1. Field of the Invention
This invention relates in general to earth boring drill bits, particularly to improved cutters for rock bits that have a high hardness on an interior bearing race and a lower hardness on the exterior tooth area to avoid tooth breakage.
2. Description of the Prior Art
A typical cutter for an earth boring rock bit is cone-shaped and contains earth disintegrating teeth on the exterior. The interior is partially hollow with bearing races to support bearings upon which each cutter rotates on the bearing pin of a head section.
It is known that higher carbon content in a steel cutter increases hardenability. To achieve high carbon content, normally a cutter carburized, then hardened and tempered. These processes act on the entire cutter, providing a fairly uniform degree of hardness and carbon content both on the interior and exterior.
Long bearing life is essentially and therefore the carburizing process of necessity produced a high carbon content. After hardening and tempering, excessive hardness in the exterior tooth area caused brittleness, occassional fatigue and tooth breakage.